bird-mammal common ancestor

[mel turner]

In article <mwspitze-110994140419 at phar2.medsurge.hsis.uci.edu> mwspitze at uci.edu (matt spitzer) writes:
>> A question for evolutionary biologists:
 >> What is the current thinking on the common ancestor of birds and mammals? 

>So far I have received a number of informative responses.  To summarize
>them, it is currently thought that mammals diverged prior to development of
>anything like a modern reptile (from which birds diverged much later). 
>Does anyone disagree?  

Basically right;  the Synapsid- therapsid-mammal line is thought to be a 
sister group to the other known Amniota (at least all living ones).    The 
main  problem is that there is no definable group "reptiles"  that means 
anything more than "those amniotes that are not mammals or birds".   

The basal branches of the mammal line are usually called the "mammal-like 
reptiles".   Among living groups, turtles appear to be a basal branch of the 
other main line, then come a group with Sphenodon, lizards and snakes, and 
finally the crocodilians as the living sister group to  birds.  (The tree gets 
a lot busier if you include fossil groups).  Birds are a branch of a branch 
the dinosaurs;  they belong to a larger group Archosauria including the crocs, 
pterosaurs etc.;  archosaurs and lizards, etc. comprise a larger group 
Diapsida,  and so forth.  "Reptiles" should just be used as an informal term 
for non-mammal, non-bird amniotes.

With regard to your earlier question about which living species is closest to 
the common ancestor,  it helps to keep in mind that all of these branching, 
diverging lineages have been evolving along their own paths  for exactly the 
same amount of time and it is unlikely that any one of them would  remain 
primitive in all respects.  ("primitive" =  the characteristics of  the  msot 
recent common ancestor of a group).   It is misleading to pigeonhole whole 
living organisms as either primitive or advanced--- (e.g., in many specific 
skeletal characters most mammals are  more primitive than frogs, turtles 
or snakes).  It is much  more meaningful to try to analyze the organisms 
character- by- character in order to reconstruct  what the most recent common 
ancestor at each branch point  of the tree must have been like.

mdt